This invention pertains to mobile positioning using a network of satellites, specifically to computing a mobile position by predicting satellite position using almanac information.
A satellite positioning system comprises a network of earth orbiting satellites to transmit geographical position information to mobile or fixed receivers.
Each satellite orbits the earth approximately once every twelve hours. The position of each satellite at any given time is precisely known and is continuously transmitted to earth. This position information, called ephemeris data, indicates the orbital position of the satellite in space with respect to satellite positioning time.
In addition to ephemeris data, the navigational signal transmitted by each satellite includes a precise timing signal in the form of a pseudo-random code specific to each satellite.
The satellite positioning system receiver computes the time taken for the satellite""s timing signal to travel to a receiver to determine distance from the satellite. This computation is accomplished by comparing the receiver internally-generated timing signal to the timing signal received from the satellite and noting the time shift between the two signals. The difference in time resulting from the time shift is then multiplied by the speed of light to determine the distance of the receiver to the satellite.
The computed distance is called xe2x80x9cpseudo-rangexe2x80x9d because receiver timing signal may not be precisely synchronized to satellite positioning system time, and because propagation through the atmosphere introduces delays into navigation signal propagation time.
Using these two pieces of information, ephemeris data and pseudo-range from at least three satellites, position of a receiver with respect to the center of the Earth can be determined using passive triangulation.
Triangulation involves three steps. First, the position of at least three satellites in view of the receiver is determined. Second, the distance from the receiver to each satellite is determined. Third, the information from the first two steps is used to determine geometrically the position of the receiver with respect to the center of the Earth.
This triangulation method, using data from three satellites, gives a rough position of the receiver. Pseudo-range calculations are dependent on the accuracy of receiver internal clock signal to be in synchronization with each of the satellite clock signals. To assure clock synchronization, a fourth satellite must be acquired by the receiver.
The process of acquiring satellite positioning system signals from a multiplicity of satellite positioning system satellites and then computing the receiver location is time consuming. This process can take from 30 seconds up to 12 minutes.
A recent U.S. Federal Communications Commission (FCC) mandate, known as xe2x80x9cEnhanced 911xe2x80x9d (E911) requires cellular telephone carriers to have the ability to locate the position of any cellular telephone user for emergency purposes. The time to locate or xe2x80x9cTime to First Fixxe2x80x9d a user location must occur within 10 seconds and must be capable of locating indoor cellular telephone users.
The prior-art satellite positioning system methods cannot provide position information within the required time of 10 seconds.
Therefore, there is a need for a method of acquiring satellite information to compute position location within the FCC""s mandate of 10 seconds, that can detect receiver position located indoors and provide the required position accuracy. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in the prior art.
The invention improves time required to fix a mobile receiver with positioning signals from a satellite positioning system using almanac information.
In a differential satellite positioning system, a reference receiver, or geo-location server, continually acquires and stores satellite position information. A mobile receiver receives the stored satellite position information from the server using a high-speed radio frequency wireless link.
Since satellite information is acquired beforehand, time normally used to collect this data is therefore saved. The mobile receiver then acquires satellite pseudo-range signals to compute a position. The acquisition phase is much faster, as the correlation peak offset to a known time reference is accurately known, and the energy search time is shortened significantly.
In operation, the server sends to the mobile receiver, via the wireless link, approximate position within 20 km of the real position along with the approximate satellite position system absolute time.
Using the approximate position of the mobile receiver, the approximate satellite position system absolute time and the mobile receiver internally stored almanacs, the mobile receiver computes a tentative list of visible satellites.
The mobile receiver then sends to the server, via its wireless link, information relating to which satellites are in tentative visibility list and identification of corresponding almanacs.
The server correlates this data with internally stored almanacs to determine if there is a position or time error as compared to current satellite ephemeris. If there is a position or time error greater than a predetermined threshold, or with an almanac unknown or unavailable to the server, or a satellite not visible from the mobile receiver approximate location, the server sends a replacement almanac to the mobile receiver via the wireless link.
Since the satellite visibility list has been updated at the mobile receiver using replacement almanacs from the server and not from the satellites directly to the mobile receiver, the prior-art time consuming satellite acquisition phase is eliminated. The pseudo-random code offset, broadcast bit boundary offset, approximate Doppler and all information necessary to ensure fast acquisition is computed at the mobile receiver using the combination of almanacs updated by the server and almanacs currently residing the mobile receiver memory.
The mobile receiver acquires pseudo-ranges from the satellites and uses almanacs stored in memory to compute a coarse position. The coarse position is sent to the server with residuals for each satellite using a wireless link.
The server computes the position of the satellites using the same almanacs stored in the mobile receiver at the time that the coarse position was computed. Using the coarse mobile receiver position, accurate satellite ephemeris data from the base station and residuals, the server corrects the coarse location and computes an accurate location.